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Thermal extraction using radiation

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Thermal extraction using radiation


In one embodiment of the present disclosure, a device is disclosed comprising a macroscopic thermal body and an extraction structure that is electromagnetically-coupled to the thermal emitting area of the thermal body. The macroscopic thermal body having a thermal emitting area, and the extraction structure configured and arranged to facilitate emission from, or receipt to the thermal emitting area that exceeds a theoretical, Stefan-Boltzmann, emission limit for a blackbody having the same thermal emitting area as the thermal body.
Related Terms: Macro Macroscopic Retic Macros

Browse recent The Board Of Trustees Of The Leland Stanford Junior University patents - Palo Alto, CA, US
USPTO Applicaton #: #20140102686 - Class: 165185 (USPTO) -
Heat Exchange > Heat Transmitter

Inventors: Zongfu Yu, Nicholas Sergeant, Torbjorn Skauli, Gang Zhang, Hailiang Wang, Shanhui Fan

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The Patent Description & Claims data below is from USPTO Patent Application 20140102686, Thermal extraction using radiation.

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This patent document claims benefit under 35 U.S.C. §119 to U.S. Provisional Patent Application Ser. No. 61/714,557, entitled “Thermal Extraction Using Radiation” and filed on Oct. 16, 2012, which is fully incorporated herein by reference; this patent document and the Appendices A-B filed in the underlying provisional application, including the references cited therein and the appended figures, are fully incorporated herein by reference.

FIELD

Aspects of the present disclosure are directed toward facilitation of far-field thermal emissions, as well as methods and devices in, and stemming from, the same.

OVERVIEW

The understanding of far-field thermal radiation had directly led to the discovery of quantum mechanics a century ago, and is of practical importance for applications in energy conversions, radiative cooling, and thermal control.

Consistent with the Stefan-Boltzmann law, it has previously been assumed that for any macroscopic thermal emitter, the coupled (emitted to received) power within any given frequency range cannot exceed that of a blackbody with the same surface area. For any macroscopic thermal body, the upper limit how much power can be dissipated is therefore commonly considered as a universal property that is independent of its internal characteristics.

While experiments have demonstrated that two thermal bodies in close proximity to each other can have thermal conductance exceeding the prediction from the Stefan-Boltzmann law. Such an enhancement, however, is believed to be a purely near-field effect. Additionally, the absorption cross-section of a single optical antenna can exceed its geometric cross-section. Within a narrow frequency range, the power spectral density of such an emitter can significantly exceed that of a blackbody, if the emitter is compared to a blackbody with the same geometric cross-section. In this situation, the size of the object is typically comparable to or smaller than the wavelengths of the thermal radiation.

SUMMARY

In one embodiment of the present disclosure, a device is disclosed comprising a macroscopic thermal body and an extraction structure that is electromagnetically-coupled to the thermal emitting area of the thermal body. The macroscopic thermal body having a thermal emitting area, and the extraction structure configured and arranged to facilitate emission from, or receipt to the thermal emitting area that exceeds a theoretical, Stefan-Boltzmann, emission limit for a blackbody having the same thermal emitting area as the thermal body.

Aspects of the present disclosure are also directed toward thermal coupling between a macroscopic (a scale larger than the thermal radiation wavelength) thermal body and far-field medium (such as a vacuum) that significantly exceeds the corresponding blackbody of the same area, within the constraint of the second law of thermodynamics. To achieve such an enhancement, in certain embodiments, the thermal body can be configured with an internal electromagnetic density of states (DOS) greater than that of the medium, and a thermal extraction mechanism is provided to increase the contributions of internal modes to far-field radiation.

BRIEF DESCRIPTION OF THE FIGURES

Various example embodiments may be more completely understood in consideration of the following detailed description in connection with the accompanying drawings.

FIG. 1 depicts a system for facilitating thermal radiation from a macroscopic thermal body, consistent with embodiments of the present disclosure;

FIG. 2 depicts a system for facilitating the transfer of thermal radiation from a macroscopic thermal body to an external object or area, consistent with embodiments of the present disclosure;

FIG. 3 depicts a system for facilitating the removal of heat from a heat source using one or more additional heat transfer techniques, consistent with embodiments of the present disclosure;

FIG. 4 shows an example emitter formed by an open area of an absorptive cavity and an emission cone of the thermal radiation inside the dome, consistent with various aspects of the present disclosure;

FIG. 5 shows an example total thermal radiation power to far-field vacuum for the structure shown in FIG. 4B, and consistent with various aspects of the present disclosure;

FIG. 6 shows an example distribution of thermal radiation on the surface of the dome for the structure shown in FIG. 4B, and consistent with various aspects of the present disclosure;

FIG. 7 shows an example emission spectra of the structures at various angles, consistent with various aspects of the present disclosure;

FIG. 8 shows an example angular emission, as obtained by integrating the spectral density over the wavelength range of 2 to 8 μm, consistent with various aspects of the present disclosure;

FIG. 9 shows infrared images of the thermal sources maintained at a temperature of 553K, consistent with various aspects of the present disclosure;

FIG. 10 shows light originating from the source at the bottom of a semi-spherical dome escaping the dome without total internal reflection, consistent with various aspects of the present disclosure;

FIG. 11A depicts a raw emission spectrum measured from FTIR, consistent with various aspects of the present disclosure;

FIG. 11B depicts the transfer function of the system, consistent with various aspects of the present disclosure;

FIG. 12 (a-c) depicts the normalized spectra measured by FTIR, consistent with various aspects of the present disclosure;

FIG. 13 (a-f) depicts a series of experiments to determine the background emission from sample holders and ZnSe hemisphere, consistent with various aspects of the present disclosure; and

FIG. 14 shows infrared camera images for angles from 0 to 60 degrees, consistent with various aspects of the present disclosure.

While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure including aspects defined in the claims.

DESCRIPTION OF EXAMPLE EMBODIMENTS

The present disclosure is related to methods and apparatuses directed to utilization of a macroscopic thermal body having a thermal emitting area, such as the facilitation of far-field thermal emissions, as well as methods and devices related to the same.

Various aspects of the present disclosure are directed towards devices and methods that include the utilization of a macroscopic thermal body having a thermal emitting area. These devices and methods can include use of an extraction structure that is electromagnetically-coupled to the thermal emitting area of the thermal body. The extraction structure facilitates emission from the thermal emitting area. In certain instances, this facilitation of electromagnetic radiation (far-field emission where the geometric features or distances are equal or larger than the characteristic wavelength) can allow macroscopic thermal body to exceed a theoretical, Stefan-Boltzmann emission limit for a blackbody having the same thermal emitting area as the thermal body. In certain more specific embodiments, the macroscopic thermal body has internal electromagnetic density of states that is greater than the internal electromagnetic density of states of a vacuum.

The theoretical, Stefan-Boltzmann emission limit for a blackbody can be adjusted to account for different surrounding mediums (e.g., a vacuum or the atmosphere). For instance, while the theoretical limit of an object having a certain area can be different depending upon the surrounding mediums, the limit can still be determined. As used herein, comparisons of the thermal emissivity of an object and the theoretical limit can be made irrespective of any particular surrounding medium. Moreover, the far-field thermal emission for a less than perfect emissive body (non-blackbody) can be improved using aspects of the present disclosure. This improvement may not exceed the Stefan-Boltzmann emission limit for a blackbody, but would exceed the Stefan-Boltzmann emission limit when adjusted to account for the less than perfect emissive body.

For various embodiments of the devices and methods of the present disclosure, the extraction structure has an index of refraction that is greater than air. Additionally, the extraction structure, consistent with various aspects of the present disclosure, can provide enough radiation channels over the thermal emitting area to ensure that additional (or all) internal modes of the emitter can out-couple.

In certain embodiments, the extraction structure has a density of states that is larger than that of the thermal emitting area. Moreover, in certain embodiments, the extraction structure can provide accessibility to a far-field vacuum, or another far-field medium, for optical modes that receive radiation from the thermal emitting area. In these embodiments, the extraction structure provides the accessibility by way of constraints on the geometry of the extraction medium.

Turning now to the figures, FIG. 1 depicts a system for facilitating thermal radiation from a macroscopic thermal body, consistent with embodiments of the present disclosure. Macroscopic thermal body 102 is a thermally (electromagnetically) emissive body. In certain embodiments, a heat source 108 can be thermally coupled to the thermally emissive body. Heat is allowed to radiate from the thermal body 102. The amount of heat that can radiate from the thermal body 102 is limited by a number of factors; however, it was generally believed to be bound by an upper limit. This upper limit is based upon the rate of radiant heat energy emitted by a blackbody having the same area, and is sometimes referred to as the Stefan-Boltzmann law. Consistent with certain embodiments, the macroscopic thermal body 102 is configured and arranged with an internal density of states that is higher than that of the surrounding medium.

Extraction structure 104 is configured and arranged as interface between thermal body 102 and emitted thermal radiation into vacuum, air or another far-field medium. In particular embodiments, extraction structure 104 can facilitate emission from the thermal emitting area by increasing the number of internal modes that can out-couple. The extraction structure 104 can also be configured to limit the amount of radiation that it emits intrinsically (e.g., by being substantially transparent to the relevant electromagnetic radiation frequencies). Certain embodiments recognize that the extraction structure 104 can be configured and arranged with some degree of absorption (e.g., not fully transparent). Thus, even if a fraction of the radiation is absorbed on its way through the extraction structure 104 the overall emission can still be enhanced. Moreover, certain embodiments are directed toward an extraction structure 104 that is configured an arranged to absorb a significant amount of radiation and thereby functions as a heat sink (using the heat capacity of the extraction structure). This can be particularly useful for cooling (e.g., relative to the use of conductive cooling of the source). For example, certain embodiments can be configured for removal of a pulse of heat. Using a moderately absorbing extraction medium, the heat pulse can be deposited in a large volume and hence potentially with a large thermal capacity (e.g., relative to a conductive medium). Accordingly, a device can be configured to cool a heat source by absorption of the radiation in the extraction structure, combined with conduction of heat away from the surface of the extraction structure.

Aspects of the present disclosure recognize that total internal reflection of electromagnetic radiation 108 (e.g., light) can occur at the interface between the medium inside the cavity and vacuum outside. This can prevent a significant portion of the internal electromagnetic modes from coupling to vacuum. Accordingly, the extraction structure 104 can be configured and arranged to eliminate or reduce such total internal reflection. This can include controlling the curvature of the extraction structure 104 and/or the refractive index. As a non-limiting, simplistic example, the extraction structure 104 could be a hemisphere with a radius (R) and an index of refraction (ne). The thermal body 102 could be a circular disk with a radius (r) centered on the flat part of the hemisphere. As such, the elements could be arranged such that R≧ner, and thereby be sufficient to ensure that any radiation 108 originated from the open area S, when it reaches the top surface of the dome, has an incident angle less than the total internal reflection angle.

Aspects of the present disclosure recognize that each of the extraction structure 104 and the thermal body 102 can have various different shapes and configurations. While the certain shapes (e.g., spherical and/or circular objects) may lend themselves to simpler calculations, the principles disclosed herein can be applied to other shapes and configurations.

Certain embodiments of the present disclosure recognize that the physical distance 106 between extraction structure 104 and the thermal body 102 can have a significant effect on the ability to out couple modes from the thermal body 102. For instance a distance of 30 μm, can inhibit photon tunneling (but yet small enough to preserve geometrical optical lens effects) for thermal radiation with a vacuum wavelength shorter than about 30 μm. It has been discovered that effective coupling can be accomplished by keeping the distance between the thermal body 102 and the extraction structure 104 close enough to allow photon tunneling between the thermal body 102 and the extraction structure 104. Such photon tunneling can be allowed when the distance is kept significantly less than the thermal wavelength of emitted energy. Since the surrounding thermal extraction medium does not need to be maintained in physical contact with the thermal body 102, the temperature of the extraction medium can be maintained substantially independently, for example, at room temperature.

The structure of FIG. 1 can be particularly useful for cooling of various heat sources 108. The amount of heat that can be dissipated by far-field thermal radiation is limited by the effective area of the thermal body. In many environments, the size of the thermal emitter can be limited by outside factors, such as thermal emissions in a device that is subject to size constraints. Moreover, the cost of providing a larger thermal emitter area can place further constraints on the size of the thermal emitter. Accordingly, the use of the extraction structure 104 to increase the effective thermal emission capabilities can facilitate the use of smaller, more efficient and/or cheaper thermal bodies 102.

Various embodiments are directed toward a thermal light source for thermal photovoltaic devices. In such cases, the size of thermal body 102 can be kept relatively small while maintaining high emission power through the use of the extraction structure 104.

Certain embodiments can be used for cooling electronics. Designers of electronic devices often incur significant cost (including cost of design and cost of components) in cooling of electronic circuits. For instance, as processing circuits become more powerful and smaller, the ability to cool the electronics becomes more critical and difficult at the same time. Moreover, many cooling mechanisms rely primary upon cooling through convection and/or conduction. For instance, a heat sink can use conduction to remove heat from a circuit and convection to transfer the heat to the surrounding environment/air. Such cooling mechanisms can be hampered by a lack of air flow. Accordingly, cooling provided through far-field thermal emissions can be particularly useful, either alone or in combination with other cooling mechanisms.

For instance, an air-cooled heat sink that is designed to sink heat away from electronics can be modified to include a thermal body 102 and an extraction structure 104 that further facilitates radiation-based cooling. This can be particularly useful when there is limited amount of air flow to the heat sink.

The size and scale of the thermal body 102 and the extraction structure 104 can be adjusted depending upon the particular application and use. For instance, large scale cooling can be carried out using relatively large sizes for thermal body 102 and the extraction structure 104. This can be useful for any number of different environments. For instance, cooling of buildings could be facilitated using a large thermal body 102 and extraction structure 104 that is thermally coupled to the building (acting as a heat source 108). In another instance, cooling of components of building heating/cooling systems can be facilitated (e.g., whether the system is a separate air conditioning unit or a heat pump). In still other embodiments, temperature control can be provided for spacecraft, which lacks of heat transfer medium due to being located in a vacuum.

Particular embodiments are directed toward adjusting the cooling efficiency by modifying the distance between the thermal body 102 and the extraction structure 104. When additional heat dissipation is desired, the distance can be reduced so as to allow structure 104 to be close enough to allow photon tunneling. When less heat dissipation is desired, the distance can be increased so as to prevent photon tunneling. For instance, cooling of a building can be desirable in the summer months, but not in the winter months.

Aspects of the present disclosure are directed toward manufacturing a device having radiation properties that facilitate far-field thermal emissions (e.g., by exceeding the Stefan-Boltzmann law). For instance, manufacturing of the device can include a process in which the device is tested relative to the Stefan-Boltzmann law so as to confirm the effectiveness of the thermal emissions. This testing can be particularly useful for insuring that the extraction structure 104 close enough to allow photon tunneling between the thermal body 102 and/or that the macroscopic thermal body has internal electromagnetic density of states that is greater than internal electromagnetic density of states of the surrounding medium (e.g., vacuum).

FIG. 2 depicts a system for facilitating the transfer of thermal radiation from a macroscopic thermal body to an external object or area, consistent with embodiments of the present disclosure. Thermal transfer structure 202 is configured and arranged to facilitate far-field thermal radiation and thereby transfer heat from heat source 204 to external locations/objects. Enclosure/transferee body 206 can be configured and arranged to receive and absorb thermal radiation from thermal transfer structure 202. This can be useful for a variety of applications.

For instance, the use of far-field radiation allows for heat transfer without direct connection between the source of heat and the recipient of the heat. Accordingly, heat can be transferred between two objects without using (or in addition to) conduction/convection, including heat transfer in vacuum conditions, such as may be useful for satellites or other spacecraft.

FIG. 3 depicts a system for facilitating the removal of heat from a heat source using one or more additional heat transfer techniques, consistent with embodiments of the present disclosure. Thermal transfer structure 302 is configured and arranged to facilitate far-field thermal radiation and thereby transfer heat from heat source 304 to allow for cooling thereof. As a non-limiting example, heat source 304 can include a cooling element that can use one or more of conduction or convection to transfer heat to macroscopic thermal body 306. Heat can then be dissipated from macroscopic thermal body 306 using an extraction structure.



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stats Patent Info
Application #
US 20140102686 A1
Publish Date
04/17/2014
Document #
13830559
File Date
03/14/2013
USPTO Class
165185
Other USPTO Classes
International Class
28F9/00
Drawings
14


Macro
Macroscopic
Retic
Macros


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